Baryon acoustic oscillations provide a standard ruler, a structure of a fixed co-moving scale, which can be used to measure the expansion history of the universe over a wide range of redshifts[1]}. Baryon acoustic oscillations in the observed matter density power spectrum arise because all modes are initially excited with the same phase on superhorizon scales, and only the growing mode is excited. Before recombination the photons and baryons are tightly coupled to each other owing to Thomson scattering and oscillate much like ordinary sound, following horizon crossing. However, after recombination the photons free stream, causing the phase oscillations of the plasma at the moment of recombination to become frozen into the power spectrum.
The characteristic scale of the first peak of this power spectrum is the sound horizon (comoving distance that density waves in the photon-baryon fluid could have travelled until decoupling), which is strongly constrained by the CMB to be 146.8 \(\pm \)  1.8 Mpc [2]}. Due to the variations in matter density produced by the BAO, and structure formation favoring areas of over-density, the distribution of galaxies subsequently displays a spatial correlation function which corresponds to the BAO power spectrum.
This large scale structure produced by the BAO has been detected at high significance in optical galaxy surveys of the low redshift universe, and has provided an important constraint on cosmological parameters, including Dark Energy[3]}, [4]}, [5]}.
